1. Field of the Invention
The present invention relates to a magnifying observation apparatus in which an optical imaging device that can obtain an optical image with an optical observation device such as an optical microscope is added to an electron microscope such as a Scanning Electron Microscope (SEM).
2. Description of Related Art
For example, a transmission electron microscope and a scanning electron microscope are well known as a charged particle beam apparatus in which a signal obtained by irradiating an observation target specimen with a charged particle beam is detected to obtain an observation image. In the electron microscope, for example, an electron traveling direction is freely deflected and an image formation system as in an optical microscope is designed in an electro-optical manner. Examples of the electron microscope includes a transmission electron microscope that forms an image of electrons transmitted through a specimen or a sample using an electron lens, a reflection electron microscope that forms an image of electrons reflected from a specimen surface, a scanning electron microscope in which the specimen surface is scanned with a focused electron beam to form an image using secondary electrons from each scanning point, and a surface emission type electron microscope (field ion microscope) that forms an image of electrons emitted from the specimen by heating or ion irradiation (for example, see Japanese Unexamined Patent Publication No. 9-97585).
In the Scanning Electron Microscope (SEM), which is one example of the electron microscope, secondary electrons and reflection electrons generated in irradiating the observation target specimen with a thin electron beam (electron probe) are taken out using detectors such as a secondary electron detector and a reflection electron detector and are displayed on a display screen such as a CRT and an LCD, and a surface mode of the specimen is mainly observed. On the other hand, in the Transmission Electron Microscope (TEM), the electron beam is transmitted through the thin-film specimen, the electrons scattered and diffracted by atoms in the specimen at this time are obtained as an electron diffraction pattern or a transmission electron microscope image, and an internal structure of a substance can mainly be observed.
When a solid-state specimen is irradiated with the electron beam, the electrons are transmitted through the solid-state specimen by electron energy. At this time, elastic collision, elastic scattering, and inelastic scattering associated with energy loss are generated by interaction between electrons and atomic nuclei constituting the specimen. In-shell electrons of a specimen element or X-rays are excited by the inelastic scattering, and the secondary electrons are emitted to lose the energy corresponding to the inelastic scattering. An emission amount of secondary electron depends on a collision angle. On the other hand, an emission amount of reflection electrons that are scattered backward by the elastic scattering and emitted again from the specimen is unique to the atomic number. In the SEM, the secondary electrons and the reflection electrons are utilized. In the SEM, the specimen is irradiated with the electrons, and the emitted secondary electrons or reflection electrons are detected to form the observation image. A Scanning Transmission Electron Microscope (STEM) in which the detector receives light transmitted through the specimen is also well known as one type of the scanning electron microscope.
Although the electron microscopes such as the SEM, the TEM, and the STEM are effectively used in the observation at a high magnifying power, the electron microscopes do not well display at a low magnifying power. Generally, the electron microscope can perform the display of tens thousands times to hundreds thousands times or millions times at the maximum magnifying power. On the other hand, the electron microscope can perform the display of several times to tens times at the minimum magnifying power. For example, in the SEM, the observation can generally be performed at the minimum magnifying power of about 5 times to about 50 times. In the observation with the electron microscope, because the display is performed at the high magnifying power from the beginning, an observation visual field becomes extremely narrow range. Therefore, it is difficult to perform visual field search that is work to finally find a site to be observed on the specimen. Preferably, the visual field search is gradually performed from the wide visual field state, namely, the state in which the specimen is displayed at the low magnifying power to the state in which the visual field is narrowed at the high magnifying power.
In order to facilitate the visual field search of such an electron microscope, there is known a method of utilizing an optical microscope in which visible wavelength light or infrared wavelength light is used and an optical observation apparatus (optical imaging device) (for example, see Japanese Unexamined Patent Publication No. 9-97585). In the observation with the optical imaging device, the display can generally be performed at a low magnifying power of the same size or less. After the specimen is observed at the low magnifying power with the optical imaging device to roughly perform the visual field search, observation is performed with the electron microscope. In order to realize this, the electron microscope is used in conjunction with the observation optical system in which the display can be performed at the lower magnifying power. The visual field search is performed based on the display performed at the low magnifying power with the observation optical system of a CMOS camera or the like. Then, the observation is performed at the high magnifying power while the observation optical system is switched to the electron beam imaging device of the SEM or the like.
In the conventional electron microscope including the optical imaging device, the observation is performed while the optical image and the electron microscope image that are obtained by the imaging devices are contrasted with each other. At this time, for the magnifying power of the image, the image of the specimen placed on a specimen stage is desirably obtained in the same visual field at the same magnifying power by each imaging device such that the sizes of the specimens displayed in the optical image and the electron microscope image become identical.
However, because the electron microscope and the optical microscope are designed by different concepts with respect to magnifying powers, unfortunately the sizes of the display images differ from each other even if the magnifying powers have the same numerical value. In other words, since the magnifying power is based on the size of the finally outputted image, the electron microscope photograph conventionally becomes a basis in the electron microscope, whereas the monitor size becomes a basis in the optical microscope because the optical image is displayed on a monitor screen. The size also depends on a CRT monitor or an LCD monitor.
Because the electron microscope image differs from the optical image in the magnifying power computing method, the sizes of the actually outputted images differ from each other even if the magnifying powers have the same numerical value. The different image sizes are not suitable for the comparative observation, so the images are desirably displayed at the same size. Therefore, for example, it is necessary that, after the magnifying power of the optical image is converted into the magnifying power of the electron microscope image, the electron beam imaging device be adjusted to the converted magnifying power to obtain the electron microscope image. As a result, when an operator who is a user tries to obtain the optical image and the electron microscope image of the same size, the user needs to understand the difference of a magnifying power determining method in each observation device, and it has been difficult for the user to easily set the magnifying powers of the observation devices.